1. Field of the Invention
This invention relates to motor speed detecting devices, and more particularly to a device for detecting a speed of a brushless motor of at least two phases.
2. Related Art
A speed detecting device for a three-phase brushless motor as shown in FIG. 6 is known in the art. The brushless motor includes: a stator having drive coils L1, L2 and L3; and a rotor with magnetic poles. The rotor is rotated as follows: That is, a Hall element 1 applies three-phase sine wave signals to a brushless motor driver 2 in correspondence to the relative position of the stator and the rotor. In the brushless motor driver 2, the signals thus applied are suitably processed, so that currents are applied to the drive coils L1, L2 and L3, whereby the rotor is rotated.
On the other hand, the speed of the brushless motor is detected as follows: The coil terminal signals of the drive coils L1, L2 and L3 are detected by a speed detector 3, and the signals thus detected are combined to form a square wave signal for detection of the speed of the motor. The square wave signal thus formed is subjected to F/V (frequency-to,voltage) conversion by an F/V converter 4. The output of the F/V converter 4 is applied to an adder 6, where it is added to a speed reference voltage Vref. The output of the adder; i.e., the sum of those voltages is fed back to the brushless motor driver 2, to control the speed of the brushless motor.
The above-described speed detecting device suffers from the following difficulties:
In the case where the voltages of the phases are shifted by the counter electromotive force in the motor, or the detector employed in the speed detecting device is low in the accuracy of detection, the waveform of the output signal g of the speed detector 3 is shifted from that which, as indicated by the dotted lines in FIG. 7(a), is provided when it is high in the accuracy of detection; that is, the signal is deviated as indicated by a speed deviation ef provided by the adder 6 as shown in FIG. 7(b). This deviation can be eliminated by increasing the accuracy of the detector or the motor; however, the method is not practical, because it will increase the manufacturing costs of them.